1. Field of the Invention
The present invention relates to a process for preparing a propylene-tetrafluoroethylene copolymer having high molecular weight. More particularly, it relates to a process for preparing a propylene-tetrafluoroethylene copolymer having high molecular weight and low Mooney viscosity by copolymerizing propylene and tetrafluoroethylene at low temperature in an aqueous medium in the presence of a specific redox catalyst system.
2. Description of the Prior Art
Heretofore, various processes for preparing copolymers of a fluorolefin, e.g., tetrafluoroethylene, trifluorochloroethylene, hexafluoropropylene, vinylidenefluoride or vinylfluoride and an olefin, e.g., ethylene, propylene or isobutylene, have been proposed.
These copolymerization reactions in aqueous media have been disclosed in Japanese Patent Publication No. 19188/1965 and U.S. Pat. No. 3,467,635. It has also been known that propylene-tetrafluoroethylene copolymers are vulcanizable elastomers having both high heat and chemical resistance. However, the propylene-tetrafluoroethylene copolymers prepared by the conventional processes are characterized by low molecular weight and it is difficult to provide satisfactory vulcanizates of sufficient tensile strength and compression set. Conventional emulsion copolymerizations of propylene and tetrafluoroethylene at 60.degree.-70.degree. C. in a redox catalyst system comprising ammonium persulfate and sodium bisulfite yield copolymers having a number average molecular weight of only about 50,000-80,000. These low molecular weight copolymers have a high Mooney viscosity (about 150-200). Blending and molding of the copolymers in the vulcanization has been difficult resulting in a high compression set of the vulcanizates. The copolymers are therefore not suitable as a raw material for rubbers.
It has also been known that copolymers having high molecular weight can be prepared by the use of high energy ionizing radiation, in a .gamma.-ray initiated emulsion copolymerization at ambient temperature. In the conventional catalytic emulsion copolymerization, the copolymerization reaction rate (hereafter referred to as the reaction velocity) is quite slow at low temperatures, particularly lower than 50.degree. C. This is therefore not satisfactory for industrial operation. It is significant to note that the copolymerization reaction of propylene and tetrafluoroethylene has never been accomplished at temperatures lower than 50.degree. C. in an aqueous medium using a conventional catalyst system. When a conventional catalyst system is used, the resulting copolymer is difficult to disperse in an aqueous medium, to form a stable dispersion, and the resulting copolymer is easily blocked.